The present invention relates to a key interface circuit, more particularly to a key interface circuit with a reduced number of input terminals.
Keyboards and keypads are in widespread use in electronic apparatus. Each key on a keyboard or keypad operates a key switch, which closes when the key is depressed. The function of a key interface circuit, which is commonly packaged as a single integrated circuit, is to sense the closing of the key switches and generate corresponding output data signals.
Conventional key interface circuits are of the static type and dynamic type. A conventional static key interface circuit has a separate input terminal coupled to each key switch. When the key switch is open, the input terminal is biased through a resistor to a first potential, such as the power-supply potential. When the key switch is closed, the input terminal is connected through the key switch to a second potential, such as the ground potential. This arrangement produces an input signal that can be directly latched and supplied to other circuits that process the key input data.
A conventional dynamic key interface circuit operates in a generally similar mariner, but places the key switches at the nodes of a row-column matrix, connects each column of key switches to a single input terminal of the key interface circuit, and scans the key matrix by driving one row at a time to the second potential.
With a static key interface circuit, there is the problem that the number of input terminals is equal to the number of keys, which may be large. Simply to accommodate the input terminals, the key interface circuit may require a large circuit package, taking up an inconveniently large amount of space. In some cases, two or more key interface circuits may be needed to provide input terminals for all of the key switches. There is also the problem of the large amount of wiring needed to connect each key switch separately to the key interface circuit.
With a dynamic key interface circuit, the number of input terminals and the amount of wiring is reduced, but the key matrix must be scanned at a rapid rate. The scanning pulses become a source of electromagnetic interference to nearby devices such as radio receivers.